Method of forming a reflection reducing coating



Jan. 2, 1951 H. R. MOULTON- 2,536,764

METHOD OF FORMING A REFLECTION REDUCING COATING Filed April 5, 1947 COATING w M a mm MN an 3 a m w w 3 12 SURFACE INVEN TOR. HAROLD l2. MOULTON ATTflfPN V i atented Jan. 2, 1951 METHOD OF FORMING A REFLECTION REDUCING COATING Harold R. Moultcn, southbridge, Mass, assignor to American Optical Company, Southbridge, Mass, a voluntary association of Massachusetts Application April 5, 1947, Serial N0. 739,545

Claims.

This invention relates to novel methods of reducing surface reflect'c-ns and of producing articles having such surfaces thereon.

This application is a continuation in part of my copending application, Serial No. 478,980, filed March 12, 1943, now Patent No. 2,432,484, issued December 7, 1947, which was a continuation in part of my earlier application, Serial No. 452,356, filed July 25, 1942, now abandoned.

One of the principal objects of the invention is to provide novel and improved methods of reducing the reflection of light impinging upon the surface of an article whereby, in the case of a transparent article, a greater percentage of incident light will traverse said article, and in the case of an opaque article the reflectivity of said articles will be reduced.

Another object is to provide simple, efiicient and economical methods of providing articles with coatings having the above characteristics.

Another object is to provide surlace coatings of the above character which are relatively durable and permanent.

Another object is to provide new and improved methods whereby coatings of the above character may be applied to the completed articles.

Another object is to provide an article with a reflection reduction coating thereon and to provide said reflection reduction coating with a bonding coating with substantially no altering of the reflection reduction characteristic of the initial coating.

Another object is to provide a unitary coating mixture having embodied therein means for reducing surface reflections and means for rendering said coating more resistant to removal.

Another object is to prov'de a novel method of producing a coating mixture which will retain desirable working characteristics.

Another object is to provide novel methods of rendering a coated art'cle more resistant to impact, more resistant to rapid temperature changes and strong temperature gradients without damage to the coating.

Another object is to provide novel methods of initially treating the articles to be coated to render said art'cles more receptive to the application of reflection reduction coatings.

Another object is to provide a surface coating composition which may be more uniformly applied to the articles to be coated through the reducton of surface tension of the coating compound.

Another object is to provide novel methods of modifying surface coating compositions of the character described for rendering them more adaptable to particular coating conditions.

Another object is to provide novel methods of applying identification means on articles havin coated surfaces of the character described.

Another object is to provide new and improved means of fortifying surface r flection reducing coatings which inherently are readily susceptible to injury and novel means and methods of applying said fortifying means either independently of the application of the non-reflection coating or simultaneously with the applications of said nonreflectlon coating.

Another object is to provide novel methods of producing permanent fiduciary marks on an article either with or Without the resultant article having a surface reflection reduction coating thereon.

Other objects and advantages of the invention will become apparent from the following description and it will be apparent that many changes may be made in the steps of the methods shown and described without departing from the spirit of the invention as expressed in the accompanying claims.

In the drawing,

Figure 1 illustrates in diagrammatic form a highly magnified fragmentary sectional view of an article having a surface reflection reducing and transmission increasing coating on a surface thereof according to the invention;

Figure 2 illustrates a more highly magnified fragmentary cross-sectional view thereof.

In following the teachings of the invention the article l!) which is to be coated may be of a light transmitting nature such as glass plates, lenses, prisms or other optical elements and similar elements formed of plastics or artificial resins. In this particular instance, the element, as stated above, is to have light transmitting properties and finished optical surfaces thereon.

In the present instance, the articles refer ed to are particularly of the type which are adapted for use in telescope systems, field glasses, binoculars, projectors, camera lenses, periscopes, windows or for any other uses in which the light is adapted to impinge upon a surface of the article. The article may be of glass, plastic or other material. The surface l2 of said articles H), in order to reduce surface reflection and to increase light transmission, is initially cleansed, if necessary, and is then coated with a layer of discrete sub-microscopic microgranular nearly spherical substantially equi-dimensional particles l I of transparent solid anhydrous material.

One method of obtaining such particle consists of ball milling for a prolonged period of time (several weeks or months) a transparent solid material such as magnesium fluoride, strontium. fluoride, lithium fluoride, barium fluoride, calcium fluoride, quartz, cryolite, glass, corundum, etc; in a liquid inert to the particular mate ial (that is, free from solvent action on it) until the particles are sufficiently small and in the size range given hereinafter to function as described. Another method, particularly adaptable to produce such particles of silica consists of chemically reacting in a water solution a soluble silicate such as sodium silicate with an acid such as hydrochloric acid and dialyzingthe resulting gel until substantially free from electrolyte. The water then can be replaced in whole or in part by an organic solvent, such as ethylene glycol sono ethyl ether, alcohol, ethylene glycol monomet'nyl ether, etc., the concentration of solids (silica) adjusted to the desired concentration, and used to produce the type of coating described. When a uniformly dispersed colloidal suspension of such discrete, sub-microscopic, micro-"ranula-r, nearly spherical, substantially equi-o mensional, transparent solid anhydrous particles applied to the article ill in asubstantially uniform liquid layer and the liquid portion of the suspension evaporated, it has been found that the particles group themselves in mounds or irregularities substantially as shown in Fig. 1.

The surface of the article ill will then be coated with a plurality of said particles piled in the form of minute projecting porous mounds or irregularities as shown in Fig. 1 with the concentraticn of the particles increasing as the surface of the article is approached whereby a layer i produced in which the index of refraction varies from substantially unity at the outer or layer air surface thereof progressively and uniformly to index of refraction approaching that of the article itself at the inner or layer-article surface thereof. In order to reduce the light reting from the surface of the coated article, and to increase transmission of light through said article as Well as to avoid excessive haze an scattering of light, the particles must be substantially smaller than a wave length of light in size. Preferably the particles are in the neighborhood of 60S 1 or less in diameter. The particles should be substantially anhydrous, solid and isometric or nearly spherical and equidimensional in shape to permit a piling into porous mound-like clumps. For this reason, particles of a gel-like hydrated fibrous nature are not desirable inasmuch as said particles would tend to mat into a non-porous mass. The clumps of pal-tic es likewise should be spaced apart less than a wavelength of light to insure in iirnum reflection of the light. Coatings produced according to the invention have extremely low reflectivity, of the order of 0.3% per surface for white light.

Because of its chemical inertness and stability in the form of a sol a suspension of silica particles in water or other suitable solvents such as ethylene glycol monoethyl ether is most useful for this purpose. A mixture of solvents also may be used.

In instances when the coating is to be formed of silica, a commercially available silica aquasol is preferably used for obtaining silica particles having the desired characteristics. Said material has a chemical composition which comes within the following formula:

Silica particles 10 to 12% Sodium oxide less than 0.05% FezOs-l-AlzOz about 0.1% And the balance water pH about 9.5

The silica particles contained in said material are discrete, micro-granular, transparent and isometric or nearly spherical and equi-dimensional in shape, solid and anhydrous in nature, and from 600 to 625 A. in diameter.

In instances where the coating is to be formed of material other than silica, as for instance of magnesium fluoride, preferably the material is reduced to proper shape and size by prolonged ball milling as referred to above.

A solution containing from 0.1 to 6 per cent by weight of said sub-microscopic, discrete, microgranular, transparent, solid, approximately equidimensional particles in water when applied to the surface of the transparent material, it has been found by experiment, produces a coating of the nature described which is highly efficient. This coating may be applied in several ways. The lens or other articles may be immersed in the liquid, removed and immediately spun to remove excess liquid or the article to be coated may be spun and while still spinning a suitable quantity of the suspension poured upon it or otherwise applied. The speed of spinning and the concentration of the solution are the controlling factors as to the amount remaining on the surface and therefore the resultant thickness of the coating, any excess being spun or thrown off. Upon evaporation of the solvent in either case the particles 1 l remain attached to the surface 12 of the article iii. The surface then consists of a dry coating of said minute projecting .porous mound-like structures or irreuglarities that are spaced apart considerably less than the wave length of the light, with the concentration of said particles in said irregularities increasing as the surface of the article is approached whereby there is a gradual progressive increase of effective index from approximate unity at the layer-air interface to a value approaching that of the substrate at the layer-substrate interface.

The thickness of said coating may be varied over a relatively wide range and still effectively reduce surface reflections. Although in the production of the interference-type reflection reducing layers of the prior art, it was considered essential that the thickness of the layers be maintained at some odd multiple of a quarter wavelength of light, such a careful thickness control of applicants coatings is not necessary. However, the thickness of the coating does affect the color of the reflected light and consequently the transmitted light. -It, therefore, is desirable to maintain the thickness approximately at a quarter wavelength of light and in general it has been found that the most efficient layer is one which reflects a reddish purple color.

The concentration of a solution which will roduce a layer or coating of such thickness depends upon its method of application. For ex ample, if the solution containing the particles colloidally dispersed therein is poured on to a platespun at 1860 R. P. M. (revolutions per minute) or if the plate is first dipped into the solution and then spun at said 1800 R. P. M., a solution containing approximately 3% particles would produce such a layer. If the velocity of spinning were increased to 2200 R. P. M., an approximately 3.l% concentration of particles would be used. If the velocity were increased to 4000 R. P. M., an approximately solution would beused. If the velocity were decreased to 1000 R. P. M., an approximate l concentration would be used. The higher the speed of rotation of the article, the higher the concentration of the solution can be; and the slower the speed of rotation, the lower the concentration of the solution.

In the case of articles which are coated by being dipped into the solution and withdrawn continuously, the more rapid the withdrawal, the lower the concentration of the particles in the solution and, conversely, the lower the rate of withdrawal, the more concentrated the solution should be. For example, where the vrate of withdrawal is approximately four inches per minute, an approximate 4% solution would be used, while if the rate of withdrawal were increased to 12 inches per minute, the concentration of particles in the solution should be correspondingly reduced to about 2%.

If the coating is to be applied by spraying, generally a of 1% concentration has proved most satisfactory. It is to be understood that the pressure at the nozzle of the spray gun of the impelling gas, the distance of the spray gun from the article to be sprayed, the viscosity of the solution are all variations which may be independently varied but must be controlled according to the concentration of solution used. Tn general,

. however, it has been found most satisfactory to use the above mentioned concentration while varying the distance of the nozzle of the spray gun from the article according to the size of nozzle, etc.

Several other method of application should be readily apparent as by painting, swabbing or the like. In general a of 1% concentration is used in said instances. Said latter methods are, however, subjected to a certain amount of streakiness. Although where convenience of application and not appearance is important, these latter methods may be used.

Regardless of the method used, for best results the rate of withdrawal, spinning, etc. together with the concentration of particles in the solution, and the volatility of the solvent used, are all factors which should be considered so as to apply a substantially uniform layer of the solution, and to avoid much as possible, dripping, streakings and resultant color blotching or irregularities of appearance. For most eificient results, where the clipping and continuous withdrawal method is used, a rapidly evaporating solvent such as one of the alcohols would be preferred. Where the article is to be spun, solvents having a relatively low rate of evaporation, such as ethylene glycol mono ethyl ether, are best. The criterion in all cases is the appearance of the coat after the solvent has evaporated. As stated above, the most advantageous coat i one in general which appears reddish purple by reflected light. If the coating is too thick the trend is toward the blue but if the thickness of the coating is increased above that which will produce a blue cast, red, yellow, green or blue coats may result, indicating that there is some influence of thickness upon the color of the reflected light. If the coating is extremely thin the efliciency will be relatively low as to the reduction of surface reflections. As the thickness is increased to a point still under that which will produce the red at least most of the water be purple reflection, the color of the reflected light passes through a brown stage indicating that the surface reflection reduction is not at it maximum and that the thickness of the coating resulting from the evaporating of the solvent carrying the dispersed colloidal material is too thin. A little experience in one or two preliminary tests will serve to correct and overcome any difliculties which may arise because of the wrong thickness being applied. Once a method of application has been selected the easiest control consists in varying the concentration of the colloidal suspension by the addition of suitable solvents such as set forth herein. Even too thick a coating unless it greatly exceeds the thickness which will produce the red purple is still rather efficient but as a few trials will indicate the proper dilution for the given method of application it is clear that the red purple should be aimed at. For example, it may be desirable, in order to compensate for the color absorption in the transparent substrate, to refiect selectively certain portions of the spectrum whereby the sum total of the effects of absorption and reflection will be neutral or uncolored Or it may be desirable in certain of the components of an optical system to introduce an intentional slight distortion of the color of the transmitted light in order to compensate for an opposite distortion in other optical elements thereby functioning as compensating means. It should again be stressed that the method of reflection reduction described is a general one wherein the actual transparent sub-microscopic anhydrous solid micro-granular approximately equi-dimensional particles used would be selected with due regard to their chemical and physical stability. At the present time the composition of choice is silica but for special uses materials more sensitive chemically and physically might be used such as the fluorides mentioned above.

The actual particles need not be of an inorganic nature. As long as the particles fulfill the other requirements of transparency, micro-granularity and sub-microscopic size, and are solid, anhydrous and not gel-like in nature, they will also be effectiv in producing surface reflection reduction but in general the most stable material chemically would be selected. A fortifying solution or composition may be applied as a separate coating or may be mixed with the above described reflection reduction coating solution with substantially no adverse effect as to the elliciency of said reflection reducing coating.

Another surface reflection reducing solution consists of the suspension of colloidal silica in a substantially anhydrous solvent such as ethylene glycol mono ethyl ether. Of course, other liquids or mixtures of liquids may be used and the above are given only by way of illustration as they have been found workable and to have desirable characteristics. In the case of a suspension of silica in water it may be desirable to incorporate in the suspension a small quantity of a surface-tension-reducing ingredient such as a compound similar in its action to the sulphonated higher alcohols. The production of the ethylene glycol mono ethyl ether suspension of silica particles is accomplished by incorporating the desired quantity of ethylene glycol mono ethyl ether with the calculated quantity of an aqueous suspension of silica particles such as the silica aquasol referred to above. When ethylene glycol mono ethyl ether is used as the suspending means, it is desirable that removed. The

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mixture-of ethylene glycol -mono ethyl ether an'd' silica aquasolata pH of less than 530 is ac'' cordingly heated to a temperature sufficient to" evaporate the water which volatilizes more'rapidly than the ethylene glycol m'onoethyl etherwhich has a higher boilin point. If a sufficient reduction in volume has taken place- (about 10%) and it hasbeen found that sufficient water has-been removed the solution'is then made upto the original'volume by the addition" of ethylene glycol mono ethyl ether'and is then ready for use. A generally useful solution contains from- 0.1 to 6% byweight of said submicroscopic microgranular silica particles. A solution containin 3% silica particles has been found most preferable audit is termed solution A. Of course, for certain types of coatings, such as for large areas, the ultimate dilution may be greater than the original strength of the solutions. the desired extent is by the addition of hydrochloric acid to the ethylene glycol mono ethyl ether before adding the silica aquasol. Other acids may be used, such as sulphuric acid, hydrobromic acid, aceticacid, etc.

After the application of the above described coating the said coating may be readily removed from the article by wiping if the coating, for some reason, is-uneven or otherwise unsatisfactory. The coating is-very resistant to the I attackof many solvents as silica itself is extremely insoluble and inert chemically. The coatin as produced is, however, not firmly adherent to the substrate in all instances, an ad- A convenient way to lower the pH to vantage in the reclaiming of defectively coated articles.

To render said coating more resistant to wiping and handling, there is applied to the first layer of finely divided micro-granular transparent particles a transparent binder layer 13 which serves to anchor in position the discrete particles H forming the surface reflection reducing layer itself without completely filling in the spaces or pores between them. This binder layer sets and binds in place the discrete particles forming the surface reflection reducing layer itself. The binder layer does this at relatively low temperatures such as 100 C. or even at room temperature and can therefore be used for cemented optical components, plastics and the i like.

This treatment is applied by means of a di'- lute solution of tetraethylorthosilicate in an organic solvent such as ethyl acetate-denatured alcohol mixture containing a' small amount of hydrochloric acid. This solution, when applied in a manner as described above and evaporat ed and allowed to react and set, becomes -a binder for holding the minute sub-microscopicmicro-granular anhydrous solid' particles" in position a clumps orirregularities and-in firm contact with the substrate. Other organic sol vents Or mixtures ofsuchsolvents may housed; in which the ethyl silicate and the acid are soluble, such as ethylene glycol mono ethyl" ether, ethylene glycol mono methyl ether, ethyl alcohol, methyl alcohol, etcl, butyl alcohol; and/or mixtures in'suchproportions-and so se-" lected as to give desirable evaporation character= istics.

A binder coating solutionis prepared as fol-- lows:

A stock solution of from 1 to 10 percent: by volume of tetraethylorthosilicate, 0=to" 60: per cent byvolume ethyl 1 tdIOper-bent I either hydrated or unhydrated silica.

'cr other" known desirable solvents such first or base coating.

8*"- by' volume concentrated hydrochloric acid'and the balance ethyl alcohol. Denatured alcoholmaybe substituted for'the ethyl alcohol. A preferred-stock solutiorr is -45 per-cent of ethyl ace-' tate, 45 per cent ofdena-tured alcohol, 5 per cent of concentrated hydrochloric acid, and'5 per cent oftetraethylorthosilicate mixed in the order given. This gives a-clear, colorless, stable stock"solutionfterrried' solution C. An essential feature of this solution is aging for several days before use. The exact reason for this is not known but it is believed to be a partial 'decomposition of the tetraethylorthosilicate by the hydrochloric acid resulting in an organo-sol of With the composition set'forth above, an aging of from two to six days 'is desirable. The organic solvents used are given by way of illustration only. Other solvents which are miscible, such as methyl acetate, methyl alcohol, isopropyl alcohol, etc.

may be used.

This transparent binder coating does not fill in the pores between the sub-microscopic microgranular particles but simply coats and anchors each particle in" the-related position which they have assumed in the forming of the first coating Vllifl applied at proper dilution. The dilution used d'ep'endsj'upon the method of application.

For example, if the article is to be spun at approximately I000 PJJPIM a solution containing approximately 1 tetraethylorthosilicate would be used; if it were to be spun'at 2900 R. P'. M., a soluticn containing approximately 2% tetraetliylorthosilicate; while if the article were to be spun at about 4000 P. M., a solution containing about 3% tetraethylorthcsili'cate would be used. Where the article was to be dipped and continuously withdrawn at about 4 inches per minute a solution containing tetraethylorthosilicate would be used, while ifit were to be withdrawn at the rate of 12 inches per minute a solution containing about 1 woul'dbe preferred.

If it is desired to remove this fortified reflection reducing coating having the :onding coating applied to it, this can be done by the use of dilute alkaline solutions, such as dilute sodium hydroxide solution, potassium hydroxide solution or other. solutions of like nature which have a solvent effect upon such binder coats. After suitable" treatment with the alkaline cleaning solution the article is flushed with water to remove the excess alkali and then may be wiped and reprocessed,.tne article prior to reprocessing being in its pristine condition; The dilute binder coating solution is prepared from the stock solution C described above. This stock composition is diluted'fo'r'producing binder coating solution by diluting with" suitable solvents, such as ethyl acetateyethyl alcohol and mixtures of these two as isopropyl alcohol, ethyl acetate, methyl acetate, etc. until the application of one or more binder coatings" does not significantly reduce the amount of surfacereile'ction reduction produced by the It is, of course, obvious that bysuita'ble dilution'sev'e'ral successive coatings can be applied without filling in the interstic'e's between the particles and, in this manner, increasing mechanical stabilization of the first reflection reducing coating- The-binder coating 'soltition'may' be applied by the same general methods used in applying the reflection reducing or base coating.

The above is an example of a preferred coating but other binders in suitable dilution and solvents may be used such as gelatin in water, polyvinyl alcohol in water, resins, etc. in organic solvents and the like depending upon the use to which the resultant article is to be put.

It has also been found that other fortifying compositions may be used. For example, a dilute solution in water of sodium silicate may be used, the dilution being such that the reflection-reduction previously produced is not destroyed. If the solution of the sodium silicate is too strong the reflection-reduction will be interfered with. A suitable composition consists of water230 parts and sodium silicate-1 part b weight. This may be applied by dipping, spraying, spinning and the like. Upon drying the sodium silicate serves to bond the particlesto each other and to the glass or article being treated thereby giving increased resistance to wiping. If desired, after the sodium silicate solution has been applied and dried the article may be treated with dilute hydrochloric acid solution in water whereby the sodium silicate is decomposed and silica liberated which serves to bond together the silica particles which have been previously applied, the sodium chloride formed simultaneously being washed away by the water in the dilute acid solution. Other acids, of course, may be used for this purpose such as nitric, sulphuric, acetic, hydrobromic, etc.

In this case the use of a colloidal silica sol is described. The aqueous sol, while quite effective, is not as easy to apply nor as efficient, in general, as the ethylene glycol mono ethyl ether sol. The method described above for producing the ethylene glycol mono ethyl ether sol solution A; namely, the addition of freshly distilled ethylene glycol mono ethyl ether to the aqueous sol and the subsequent removal of most of the water of the aquasol while heating to approximately 50 0., although usable, if ob tained by the following method will be greatly improved upon. The heating operation for removing the moisture by following the first de scribed method had to be of long continuation and was very critical, as the mixture had a ten dency to jell or coagulate, whereby the silica particles had a tendency to separate from the solution and if great care was not exercised the resulting composition was useless and had to be thrown out.

It has now been found that it is a relatively simple matter to produce such compositions by the addition to the ethylene glycol mono ethyl ether of approximately 1% of concentrated hydrochloric acid before the addition to the ethylene glycol mono ethyl ether of the aquasol. In fact, it is possible to make a concentrated sol of the silica in ethylene glycol mono ethyl ether by the following method:

A certain volume of ethylene glycol mono ethyl ether is taken and to it is added 1% of its volume of concentrated hydrochloric acid. A quan tity of silica aquasol equivalent in volume to the ethylene glycol mono ethyl ether is then added with stirring. The resulting mixture is placed in a flask and boiled vigorously until the volume has been reduced to that of the original ethylene glycol mono ethyl ether or somewhat less. The result is a concentrated ethylene glycol mono ethyl ether sol termed solution B which can then be used as a master composition which is diluted as desired with miscible liquids, such as ethylene glycol mono ethyl ether, ethylene glycol mono methyl ether, alcohol, ethyl acetate, etc. Ethylene glycol mono methyl ether, ethylene glycol mono butyl ether, ethylene glycol mono ethyl ether acetate and similar compounds may replace ethylene glycol mono ethyl ether With advantages in some cases. Suitable mixtures of low, medium and high boiling point solvents may be used. The solvents described above are given only by Way of illustration as other organic solvents compatible with the aquasol and having boiling points above that of water may be used. Even water may be used for diluting the organo sol concentrate. The use of surface active agents, that is, materials which reduce surface tension, makes it possible under some circumstances to use the silica particles in the form of an aquasol but for most uses the organo sol, such as ethylene glycol mono ethyl ether sol is preferable. For example, a surface active material such as palm kernel oil soap may be used in the amount of from .01 to .1 of 1% by weight based on the total solution. Di octyl sodium sulphosuccinate in similar proportions is also effective.

Dilution of the concentrated ethylene glycol mono ethyl ether sol with 4 to 5 times its volume of pure ethylene glycol mono ethyl ether gives a solution corresponding to solution A described above.

Another solution found useful has the following composition:

Parts by volume Butyl alcohol Ethylene glycol mono butyl ether 10 Ethylene glycol mono ethyl ether '75 Ethyl alcohol 150 Solution B 50 Another variant when used for dipping articles having large area, such as windows, windowpanes, may be made up as follows:

30 parts of silica aquasol are diluted with 2000 parts of methyl alcohol or acetone. In place of the silica aquasol an equal quantity of the ethylene glycol mono ethyl ether sol concentrate, solution B, described above may be used. A large sheet of material may be dipped in this composition, removed immediately and allowed to drain and dry. After drying, a second coat is applied in like manner and drained in the opposite direction to equalize the coating. For fortifying the coating, that is, rendering it more resistant to abrasion, this may be followed by a single dip consisting of 2000 parts acetone or alcohol and 200 parts of an aged tetraethylorthosilicate composition consisting of 45 per cent denatured alcohol, 50 per cent ethyl acetate, 5 per cent hydrochloric acid and 5 per cent tetraethylorthosilicate, solution C. After having been dipped in this last composition, removed as above, and allowed to drain and dry, the article is then heated to a temperature from 50 to C. for about an hour. If the art cle is of such a nature that it can be heated to a higher temperature than 100 C., the temperature to which it is heated may be raised to any desired extent within practical limits and the time correspondingly shortened.

It has been found that after the application of a si ica organo sol to the surface of an article, in this case. glass, fol owed by the fortifying layer, the article may be advantageously heated nearly to the softening point of the glass. This is parfor a period of 1 to 5 minutes depending ticularly desirable in the case of protection lenses which are heat treated to increase their resist- .treated toincrease their resistance .to rapid tem perature changes and strong temperature gradients. When treated in this wa theresistance of the coating is greatly increased over coatings which have been heat treated only to the temperature of 100 C. A usual temperature for such heat treatment might be as high as 500 .to 800C. It is usual to heat such articles to this temperature upon the thickness of the article and its other dimensions. The hardening or toughening is accomplished by subjecting the. articles to .a blast of cool air or other gaseous media.

It has also been found that the fortifying layer can be formed simultaneously with the reflectionreducing layer by glycolrnono ethyl ether-sol-containing solutions. The proper quantity to be added depending upon the method of application but in all instances the amount being suflicient to produce the-fortifying effects desired without appreciably reducing the reflection-reducing characteristics. For example, a solution for both the production of a reflection reducing coating and for simultaneously for tifying said coatingmay be formed -substa ntially as follows:

Parts byvolume Solution A 100 Ethyl alcohol 18 Ethyl acetate l8 Tetraethylorthosilicate 2 Hydrochloric acid 2 The proportions of the above may be varied depending upon the use and method of application as described above.

Another solution ma be formed substantially as follows:

Parts by volume Butyl alcohol M 160 Ethylene glycol mono butyl ether Ethyl alcohol 450 Solution 13 100 Solution C 50 In general, the preferred solutions would have formulae falling within the following approximate range of proportions:

Parts by volume thylene glycol mono ethyl ether 97 to 13-3 Ethyl alcohol 0 to 18 Ethyl acetate '0 to 18 Tetraethylorthosilicate lto 3 Concentrated hydrochloric e s lto 3 applied to a glass surface that-writin upon it with a metal point or with a pencil produces a mark. "Upon wiping off the unfortified coating the addition of a tetraethylorthosilicate containing composition to the ethylene the mar-l; is not removed but remains as a permanent markon the glass surface. Identification or other desirable markings ma be made in this manner. This may also lend itself to the production of reticules. The mark ma be-ap plied to 'the fortified coating, both coating and mark being then relatively permanent.

For spraying the sol it is desirable to hav a suitable balance of solvents. While the ethylene glycol mono ethyl ether sol in proper dilution with ethyleneglycol mono ethyl ether is usable, the-addition of other ingredients, such as higher boiling miscible solvents such as butyl alcohol, ethylene glycol mono butyl ether, ethylene glycol mono ethyl ether acetate and the like and the addition of more volatil solvents such as ethyl alcohol etc. as above stated produces better sprayability whereby a mor uniform coating is obtained with the usual spray gun.

It has been found that the application of a layer of the nature described to the surface of a photographic print on paper or other material results in a great improvement in the range of lights and shades delineated on the photographic print. Thecrdinary photographic print ranges from perhaps reflection of the incident light (in the white portion of the picture) to perhaps 10% reflection in the black portions of the pic ture. By treatment, as described above, the black becomes blacker because the diluting renected white light from the-more or less glossy surface of the paper is substantially eliminated and the print then takes on the appearance of a platinum bromide print.

The application of the base coating ofthe nature described for surfaces of a photographic print is particularly-effective in the case of photographic prints made on matte paper such as is used for the highest grade photographic prints. It is not generally recognized how much diluting white light is reflected from the black portions of a photographic print or matte paper until a coating of the nature described has been applied for comparative purposes. Of course, the coating is also highly effective on lossy prints.

The application of such a surface reflection reducing layer is also extremely effective upon oil paintings whereby again the diluting white light reflection is substantially eliminated and the true values of the colors are visible to an observer without the necessity of elaborate lighting arrangements. In fact, even under the most elaborate lighting arrangements the effect is not as satisfactory as in the case where the above described reflection reducing coating is applied because-of the roughness of texture of the surface of the oil painting, which renders it impossible to eliminate all reflection by modification of either direct or indirect light.

The surfaces of maps protected byglossy coatings may also be made less reflectin and more readily visible by meansof the coating described above, with the said coating functioning also as a preservative eitherfor the oil paintings or maps or any articles to which it may be applied.

In certain-optical instruments it is desired to obtain the effect ,of front surface mirrors. Ordina-rily this requires .:the reflecting coating to be placed on the front of the article. However, the use of a-transparent material with the reflecting surface On the back but having a coating'of the nature described above on the front surface'becomes in effect a front surface mirror in that the annoying and. objectionable reflections from the transparent front surface are substantially eliminated whereby the double image effect is removed.

It is generally considered that dispersions of solids and liquids, customarily called colloidal solutions, are not true solutions. For convenience, applicant uses the term solution for such c01- loidal suspensions of microscopic, micro-granular, isometric particles in liquids. The mixtures which are called solutions are liquids filterable under certain conditions and for all practical pur poses may be considered as solutions. A very wide variation in the actual solvent composition may be made in order to adapt the solutions for use with widely varying methods of application by following the teachings set forth herein. Stock solutions of the coating mixture disclosed herein may be provided in which instances solvents may be added according to the particular intended use.

From the foregoing description, it will be ap parent that simple, efficient and economical means and methods have been provided for accomplishing all of the objects and advantages of the invention.

I claim:

1. The method of forming on the normally light reflecting surface of an article a surface reflection reducing coating having an index of refraction progressively increasing in value from approximately unity at its air-interface to a value approaching that of the article at its article-interface, said method comprising the steps of applying onto the surface of said article a substantially uniform thin layer of a volatile liquid containing substantially uniformly dispersed therein a plurality of discrete, sub-microscopic, microgranular, approximately isometric, transparent particles of solid, substantially anhydrous, inorganic material having an index of refraction greater than unity, said particles being of a diameter less than 625 angstroms and substantially insoluble in the liquid, and the liquid being substantially inert in its effect on the particles and on said surface of the article, and allowing said liquid to evaporate to deposit the particles on said surface of said article, while retaining the particles substantially unchanged in their physical character throughout the process, the thickness of the liquid layer applied being controlled according to the concentration of the particles in said layer to produce a resultant dry layer of said particles piled in a plurality of mound-like p jections to approximately a quarte wave-lengt in thickness whereby a surface reflection reducing coating of the desired character results.

2. The method of forming on the normally light reflecting surface of an article a surface reflection reducing coating having an index of refraction progressively increasing in value from approximately unity at its air-interface to a value approaching that of the article at its articleinterface, said method comprising the steps of applying onto the surface of said article a substantially uniform thin layer of a volatile liquid containing substantially uniform y dispersed therein from a proximately .1 to 6.0 per cent by weight of discrete, sub-microscopic, microgranular, approximately isometric, transparent parti cles of solid, substantially anhydrous silica, said silica particles being of a diameter less than 625 angstroms and substantially insoluble in the liquid, and the liquid being substantially inert in its effect on the particles and on said surface of the article, and allowing said liquid to evaporate to deposit the particles on said surface of said article, while retaining the particles substantially unchanged in their physical character throughout the process, the thickness of the liquid layer applied being controlled according to the concentration of the particles in said layer to produce a resultant dry layer of said particles piled in a plurality of mound-like projections to approximatel a quarter wave-length in thickness whereby a surface reflection reducing coating of the desired character results.

3. The method of forming on the normally F ht reflecting surface of an article a surface reflection reducing coating having an index of refraction progressively increasing in value from approximately unity at its air-interface to a value approaching that of the article at its article-interface, said method comprising the steps of applying onto the surface of said article a substantialiy uniform thin layer of a volatile liquid containing substantially uniformly dispersed therein a plurality of discrete, sub-microscopic, rnicrogranular, approximately isometric, transparent particles of solid, substantially anhydrous, inorganic material having an index of refraction greater than unity, said particles being of a diameter less than 625 angstroms and substantially insoluble in the liquid, and the liquid being substantially inert in its effect on the particles and on said surface of the article, and allowing said liquid to evaporate to deposit the particles on said surface of said article, while retaining the particles substantially unchanged in their physical character throughout the process, the thickness of the liquid layer applied being controlled according to the concentration of the particles in said layer to produce a resultant dry layer of said particles piled in a plurality of mound-like pro- .iections to approximately a quarter wave-length in thickness, and coating said particles with a thin film of transparent bonding material to bond the particles to each other and to the surface of the article while maintaining interconnecting interstices therebetween whereby a surface reflection reducing coating of the desired character results.

4. The method of forming on the normally light reflecting surface of an article a surface reflection reducing coating having an index of refraction progressively increasing in value from approximately unity at its air-interface to a value approaching that of the article at its article-interface, said method comprising the steps of applying onto the surface of said article a substantially uniform thin layer of a volatile liquid containing substantially uniformly dispersed therein a plurality of discrete, sub-microscopic, microgranular, approximately isometric, transparent particles of solid, substantially anhydrous, in organic material having an index of refraction greater than unity, said particles being of a diameter less than 625 angstroms and substantially insoluble in the liquid, said liquid further containing minor amounts of a decomposable silicon compound solube in the liquid, the liquid being substantially inert in its effect on said decomposable silicon compound, the particles and said surface of the article, and allowing said liquid to evaporate to deposit the particles on said surface of said article and to simultaneously cause the silicon compound to decompose and produce a thin film of silica on the particles, while retaining the particles substantially unchanged in their physical character throughout the process, the thickness of the liquid layer applied being controlled according to the concentration of the particles in said layer to produce a resultant dry layer of said particles "piled in a plurality of mound-like projections to approximately a quarter wave-length in thickness-and the proportion. of decomposable silicon compound -being con trolled to permit the film of silica produced on the particles to be sufficient to bond the particles to each other and to the surface of the article while maintaining interconnecting interstices therebetween, whereby a surface reflection reducing coating of the desired character results 5. The method of forming on the normally light reflecting surface of an article a surface reflection reducing coating having an index of refraction progressively increasing in value from approximately unity-at its air-interface to a value approaching that of the article at its article-interface-said method comprising the steps of applying onto the surface of said article a substantially uniform thin layer of a volatile liquid consaid surface of said article, while retaining the particles substantially unchanged in their physi cal character throughout the process, the thick- 16 ness ofthe-liquidlayer applied being controlled according to the concentration of the particles in said layer to produce a resultant dry layer of said particles. piled in a plurality of mound-like projections to approximately a quarter wavelength in thickness, applying to said layer of particles a relatively thin coating of a composition embodying a decomposable silicon compound dissolved'ina'volatile solvent that is substantially inert-in its effect-on the decomposable silicon compound, the layer of particles and the surface of the article, and allowing said solvent to evaporate to cause the silicon compound to decompose and produce a thin film of silica on said particles, the proportion of decomposable silicon compound being-controlled to cause said silica film produced to be eufiicient to bond the parti cles to each other and to the substrate while maintaining interconnecting interstices therebetween whereby a surface reflection reducing coating of the desired character results.

HAROLD R. MOULTON.

REFERENCES CITED The following references are of record in the file of'this patent:

UNITED STATES PATENTS Number Name Date Re. 22,076 Cartright Apr. 21, 1942 1,922,254 McCulloch Aug. 15, 1933 2,338,233 Dimmick Jan. 4, 19% 2,366,516 Geflcken Jan. 2, 1945 

4. THE METHOD OF FORMING ON THE NORMALLY LIGHT REFLECTING SURFACE OF AN ARTICLE A SURFACE RAFLECTION REDUCING COATING HAVING AN INDEX OF REFRACTION PROGRESSIVELY INCREASING IN VALUE FROM APPROXIMATELY UNITY AT ITS AIR-INTERFACE TO A VALUE APPROACHING THAT OF THE ARTICLE AT ITS ARTICLE-INTERFACE, SAID METHOD COMPRISING THE STEPS OF APPLYING ONTO THE SURFACE OF SAID ARTICLE A SUBSTANTIALLY UNIFORM THIN LAYER OF A VOLATILE LIQUID CONTAINING SUBSTANTIALLY UNIFORMLY DISPERSED THEREIN A PLURALITY OF DISCRETE, SUB-MICROSCOPIC, MICROGRANULAR, APPROXIMATELY ISOMETRIC, TRANSPARENT PARTICLES OF SOLID, SUBSTANTIALLY ANHYDROUS, INORGANIC MATERIAL HAVING AN INDEX OF REFRACTION GREATER THAN UNITY, SAID PARTICLES BEING OF A DIAMETER LESS THAN 625 ANGSTROMS AND SUBSTANTIALLY INSOLUBLE IN THE LIQUID, SAID LIQUID FURTHER CONTAINING MINOR AMOUNTS OF A DECOMPOSABLE SILICON COMPOUND SOLUBLE IN THE LIQUID, THE LIQUID BEING SUBSTANTIALLY INERT IN ITS EFFECT ON SAID DECOMPOSABLE SILICON COMPOUND, THE PARTICLES AND SAID SURFACE OF THE ARTICLE, AND ALLOWING SAID LIQUID TO EVAPORATE TO DEPOSIT THE PARTICLES ON SAID SURFACE OF SAID ARTICLE AND TO SIMULTANEOUSLY CAUSE THE SILICON COMPOUND TO DECOMPOSE AND PRODUCE A THIN FILM OF SILICA ON THE PARTICLES, WHILE RETAINING THE PARTICLES SUBSTANTIALLY UNCHANGED IN THEIR PHYSICAL CHARACTER THROUGHOUT THE PROCESS, THE THICKNESS OF THE LIQUID LAYER APPLIED BEING CONTROLLED ACCORDING TO THE CONCENTRATION OF THE PARTICLES IN SAID LAYER TO PRODUCE A RESULTANT DRY LAYER OF SAID PARTICLES PILED IN A PLURALITY OF MOUND-LIKE PROJECTIONS TO APPROXIMATELY A QUARTER WAVE-LENGTH IN THICKNESS AND THE PROPORTION OF DECOMPOSABLE SILICON COMPOUND BEING CONTROLLED TO PERMIT THE FILM OF SILICA PRODUCED ON THE PARTICLES TO BE SUFFICIENT TO BOND THE PARTICLES TO EACH OTHER AND TO THE SURFACE OF THE ARTICLE WHILE MAINTAINING INTERCONNECTING INTERSTICES THEREBETWEEN, WHEREBY A SURFACE REFLECTION REDUCING COATING OF THE DESIRED CHARACTER RESULTS. 